1. Field of the Invention
The present invention generally relates to a piezoelectric actuator. More specifically, the present invention relates to a piezoelectric actuator for driving a driven member by the oscillation of a piezoelectric element, and a device containing this piezoelectric actuator.
2. Background Information
Piezoelectric actuators that utilize the oscillation of piezoelectric elements have few structural elements, can be reduced in size to be suitable for micromachining, and are used as drive sources for miniature lens drive mechanisms, date display drive mechanisms, and other such small devices.
Piezoelectric actuators drive rotors and other driven members that contact a piezoelectric element of the actuator as a result of oscillation. For example, in a piezoelectric actuator that utilizes both a longitudinal oscillation mode and a flexural oscillation mode, a contact section is moved elliptically to drive a rotor. At this time, when the contact section is moved elliptically with the longitudinal oscillation mode as a primary oscillation mode, the rotational speed and torque of the rotor reach their maximum points.
The longitudinal oscillation mode and the flexural oscillation mode have different resonance frequencies (frequencies at which displacement is at its maximum); therefore, the contact section can be made to move in the optimum elliptical pattern by oscillating the piezoelectric element at the optimum drive frequency between the resonance frequencies.
However, piezoelectric actuators have different individual optimal drive frequencies due to nonuniformities in their shapes. Therefore, the drive frequency of each piezoelectric actuator must be controlled in order to drive reliably the piezoelectric actuator in a stable manner. The optimal drive frequencies also vary depending on the surrounding temperature and other such factors.
Known methods in conventional practice include a method wherein a detection electrode is provided at the position of the piezoelectric element in which strain is most likely to be induced by the force from the rotor, which is the driven member. Further, the drive frequency is controlled by utilizing the fact that the frequency at which the voltage detected at this position reaches its peak, and the frequency at which the rotational speed reaches its peak are in substantial agreement with each other. In an alternate method, the phase difference between the drive signal and the detection signal is determined and the drive frequency is controlled, as shown in Japanese Laid-Open Patent Application No. 2002-291264, especially pages 9–10 and FIG. 15. Japanese Laid-Open Patent Application No. 2002-291264 is hereby incorporated by reference.
Further, a detection electrode may be provided at the position at which the detection voltage of the longitudinal oscillation mode and the detection voltage of the flexural oscillation mode both increase, and both of the detection voltages are determined. In another known method, the drive frequency of the piezoelectric actuator is controlled using a similarity between the frequency at which the multiplied value of the detection voltages (root-mean-square value) reaches its peak and the frequency at which the rotational speed of the rotor reaches its peak, as shown in Japanese Laid-Open Patent Application No. 2003-304693, especially pages 6–8 and FIG. 12. Japanese Laid-Open Patent Application No. 2003-304693 is hereby incorporated by reference.
However, in the aforementioned prior art documents, the method of controlling the drive frequency on the basis of the detection voltage value has problems in that this voltage value fluctuates in size depending on whether the force from the rotor acts on the piezoelectric actuator, and is also susceptible to noise.
Also, No. 2002-291264 discloses a method of utilizing the phase difference between a detection signal and a drive signal, but in the frequency characteristics of the phase difference between the drive signal and the detection signal at the position of the detection electrode shown therein, the phase difference that originates in the longitudinal oscillation mode and the phase difference that originates in the flexural oscillation mode have the same sign (either positive or negative) and substantially the same magnitude.
For example, FIG. 15 shows the phase difference characteristics of the piezoelectric actuator in No. 2002-291264 in relation to the drive frequency. The piezoelectric actuator has a longitudinal oscillation mode extending in the longitudinal direction of the piezoelectric element, and a flexural oscillation mode in which bending occurs in the direction substantially orthogonal to the oscillation direction of the longitudinal oscillation mode. Further, the phase difference increases near the resonance frequencies f1 and f2 of oscillation in each oscillation mode, as shown in FIG. 15. Therefore, a plurality of drive frequencies (three drive frequencies fb1, fb2, and fb3 in FIG. 15) exist in relation to the phase difference θ0 to be controlled.
In such cases, the drive frequency is controlled so that the phase difference is brought to a specific value θ0, but it is sometimes impossible to set the drive frequency to a single value and the appropriate oscillation component ratio of the oscillation modes cannot be obtained, which compromises the reliability of the drive performance of the piezoelectric actuator.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved piezoelectric actuator and device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.